In an assembled battery having a plurality of energy storage devices connected in series, charge amounts of the energy storage devices may often vary (a charge amount difference may be caused) due to such reasons as differences in initial capacities and degrees of deterioration between the energy storage devices. When a charge amount difference is produced between the energy storage devices, overvoltage occurs during charging in an energy storage device whose charge amount is relatively large. Therefore, conventionally, there is known a technique for equalizing charge amounts by estimating a charge amount difference between energy storage devices, and by discharging (or charging) each of the energy storage devices based on the charge amount difference (JP 2011-41452 A). JP 2011-41452 A discloses a method of obtaining time points at which voltages of the corresponding energy storage devices reach a specific voltage, and estimating the charge amount difference based on differences between the time points.
However, according to the method of JP 2011-41452 A, the specific voltage is set to an arbitrary static value. This leads to a problem that a voltage of an energy storage device may often not reach the specific voltage depending on the setting value of the specific voltage, and a charge amount difference of the energy storage devices may not be estimated.